3.8.2Additional lessons
Energy Flow in Ecosystems — IB Biology SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 92 of 142
Part 1 of 6Flow of Chemical Energy
Energy enters ecosystems at producers and leaves as heat.
1
Producers use light energy to make carbon compounds such as carbohydrates, lipids and amino acids.
2
A primary consumer eats the producer and digests and absorbs the carbon compounds, which contain stored chemical energy.
3
Secondary consumers feed on primary consumers, passing the energy along the food chain.
4
At each stage the organism uses some energy in respiration and releases the rest as heat.
5
Dead organisms and waste pass energy to detritivores and saprotrophs.
Part 2 of 6Food Chains and Food Webs
Food chain
- Food chain A sequence showing feeding relationships, with each arrow pointing from the organism eaten to its consumer.
Arrow meaning
- Arrow meaning The arrow shows the direction of energy and biomass transfer.
Trophic level
- Trophic level The position of an organism in a food chain, starting with producers.
Food web
- Food web A diagram of linked food chains, because most organisms feed on, and are food for, more than one species.
Decomposers
- Decomposers Detritivores and saprotrophs recycle nutrients and release energy through respiration.
Part 3 of 6Energy Losses
Respiration
- Respiration Organisms oxidise carbon compounds for ATP, and much of the energy is released as heat.
Uneaten parts
- Uneaten parts Bones, roots and shells may not be eaten.
Egestion
- Egestion Material that is not digested passes out as faeces.
Result
- Result Only a small proportion of energy passes to the next level, which limits the length of food chains.
| Level | Energy / kJ m⁻² year⁻¹ |
|---|---|
| Producers | 20 000 |
| Primary consumers | 1 600 |
| Secondary consumers | 150 |
Part 4 of 6Efficiency of Transfer
Efficiency of Transfer
Formula
- Formula Efficiency = (energy in the next level ÷ energy in the previous level) × 100.
Typical values
- Typical values Transfer is often around 10%, but it varies between ecosystems and trophic levels.
Short chains
- Short chains Because energy decreases at each step, food chains rarely have more than four or five trophic levels.
Eating lower
- Eating lower A higher proportion of the original energy reaches humans eating plants than eating meat.
Part 5 of 6Worked Example
Question
- Producers in an ecosystem contain 20 000 kJ m⁻² year⁻¹ and primary consumers contain 1 600 kJ m⁻² year⁻¹.
- Calculate the efficiency of energy transfer.
- Energy in next level = 1 600 kJ m⁻² year⁻¹.
- Energy in previous level = 20 000 kJ m⁻² year⁻¹.
- Efficiency = (1 600 ÷ 20 000) × 100 = 8.
Answer 8%
Part 6 of 6Exam Checklist & Tips
3.8.2 – Key Points for the Exam
- Trophic level The feeding position of an organism in a food chain.
- Food web A network of interconnected food chains.
- Producer An organism that makes organic compounds from inorganic ones.
- Saprotroph An organism that obtains nutrients from dead organic matter.
Common Mistakes
- Drawing arrows in a food chain from predator to prey.
- Saying energy is recycled in food chains.
- Forgetting heat loss from respiration as a main cause of energy loss.
- Stating that exactly 10% is transferred in every case.
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